Showing Dark of the Sun by Chelsea Quinn Yarbro

It is the 6th century of the common era. The vampire Saint-Germain, known in this time as Zangi-Ragozh, is peacefully doing business in Asia when the island of Krakatoa explodes in a massive volcanic eruption. Tidal waves swamp harbors hundreds of miles away, destroying trade ships and their cargoes; tons of ash and dirt are flung into the air.

In the months to come, the world grows colder and darker as the massive volcanic cloud spreads across the globe, blocking sunlight. Sea trade is ravaged. Crops fail. Livestock, and then people, begin to starve. Disease spreads. Panic rises.
As always in times of trouble, foreigners-and the vampire is always a foreigner-become targets. Fleeing toward the West, where he hopes to find safety and sanity, the vampire travels with a nomadic tribe led by Dukkai, a female shaman who soon becomes Zangi-Ragozh's lover.

But Zangi-Ragozh's problems are far from over. His vampire nature is discovered by an enemy; he begins to starve; he is betrayed by one he cared deeply for; he loses everything, even his last sack of his native soil.

With the True Death looming, Zangi-Ragozh tries to reach sanctuary in his ancient homeland.
Transylvania.